This is a very common doubt but unfortunately there is no fixed answer for it. If you want to find how long you need to study for GRE then you should do the following. Take one of the two GRE Powerprep downloadable test(you need to fill up the form before you download it) which are available in the official site or you should try out one of the full length practice tests which are given in the GRE Barrons book.The time needed to study for GRE will depend on what marks you get in the practice test and how easily you can improve if needed. Just take one powerprep test now, you should take the second one right before the actual exam.
Depending on the score you get on this practice test you should decide how long you will need to practise to get your score as score to 800 in each section as possible. You should analyze the score section wise and then based on how fast you can improve you should give yourselves time accordingly.
Finding your weak areas in GRE:
Ideally you should take this weakness identifying test about five months before the date. Once you take the first test to assess yourself make a note of what kind of problems you are good at and which ones you got wrong. If you are weak in geometry questions then you should spend more time reviewing geometry problems.
The next section will focus on how to study for your weaker section. It could be either quantitative or verbal. Depending on the major you want to study in you should focus more on the section whose scores are important to you during the admission process. For engineering students quantitative scores are given more importance whereas language majors would do well to perform well in the verbal section. Concentrate more on the section that is given weightage for admissions to your particular course..